In refrigeration systems, frost tends to build up on the evaporator or cooling coils due to condensation of the moisture in the air cooled by the coils. This frost build-up reduces the efficiency of the heat transfer, and when substantial, creates a significant resistance to air flow across the cooling coils. It is necessary, therefore, to defrost the cooling coil surfaces periodically so that it can be restored to its original frost-free condition and operate in its normally efficient manner.
Changes in ambient conditions, such as differences in the moisture content of the air, varying numbers of times doors to the cooling or refrigeration compartments are opened, variance in the amount of humidity in the air on given days, and the diverse applications of cooling systems in different environments results in significant variations in the amount of frost as a function of time, both from system to system and for any one system subjected to such varying conditions.
Prior art devices have incorporated time controls which arbitrarily fix the frequency at which defrosting occurs. These time control devices do not take into account the actual amount of frost that is present on the cooling coils. Since such systems are preset to initiate a defrost cycle at fixed time intervals without regard to the actual need for defrost operations, defrost cycles may commence before there is a real need for defrosting, or well after a time when a defrost cycle should have been started. In either case, the result is a significant waste of electrical energy.
When the defrost cycle does not occur as frequently as needed, the efficiency of the cooling system is greatly reduced and more energy is expended running the compressor more than is needed. When the defrost system operates more frequently than is needed, energy is also wasted due to the excessive energy needed to power the heaters which heat up the cooling coil surfaces, and to recool the system after defrosting is complete. Keeping a time control device in adjustment for the specific application and variable weather and humidity conditions would require continual maintenance and adjustment and is not really a practical alternative.
Further, one device which presently attempts to control defrosting by demand requires installation of a separate sensing unit mounted on the cooling coils. Such units sense only the frost build-up on themselves, and thus are designed to simulate the frost build-up on the cooling coil. Such an indirect method has not proved to be an accurate way to detect the actual frost that builds up on the cooling coil, not only because it attempts to detect frost indirectly, but also because it is restricted to sense frost only at a single point.
Another approach for detecting frost contemplated a fan motor sensing circuit to sense variations in the fan speed as a result of air flow resistance due to the frost build-up. Attempts to construct this type of system have met with little success.
Air temperature sensing devices to detect the need for defrosting have also been tried. Such a device would clearly only detect the temperature in the vicinity of the heat exchanger and not the amount of frost actually accumulated on the heat exchanger. This indirect method of detecting frost build-up has been generally unreliable or at least too variable to serve its intended purpose.